The Hodges Residence is a superinsulated direct gain passive solar home in Ames, Iowa (latitude 42.0 N, longitude 93.8 W). The architect was David A. Block, A.I.A., and the solar system was designed by David Block and Laurent Hodges. The home was designed in 1978 and built in 1978-1979. For its design, David Block was a winner in the 1978 National Passive Solar Residential Design Competition. A notable feature of the home was its innovative use of concrete cored slabs as heat storage. The home has proved to have a very low Home Heating Index of less than 0.7 W/K per square meter (3 Btu/F-day per square foot). The performance of the home during its first ten years of occupancy, its economics, and occupant reactions are described.


The Hodges Residence is a partially earth-sheltered superinsulated direct gain passive solar home in Ames, Iowa (latitude 42.0 N, longitude 93.8 W). The architect was David A. Block, A.I.A., who is professor of architecture at Iowa State University, Ames, Iowa, and the designer of numerous passive solar homes since the Hodges Residence, which was his first completely passive solar home design. The passive solar system was designed by Block and the owner, Laurent Hodges. Block's design of the home was one of the winners in the 1978 National Passive Solar Residential Design Competition sponsored by the U. S. Department of Housing and Urban Development and the U. S. Department of Energy. The home was featured in Passive Solar Architecture: Logic and Beauty: 35 Outstanding Houses Across the United States by David Wright and Dennis A. Andrejko (New York: Van Nostrand Reinhold, 1982).

The Hodges Residence was the first scientifically-designed passive solar home in Iowa. It was intended by the designers to serve as a demonstration of the state of the art of energy-efficient house design and construction for the state of Iowa; it has met this goal very successfully and continues to serve as a model, being discussed in detail at many residential energy conservation workshops held in Iowa through Iowa State University Extension.


The home is on two levels, each with a floor area of about 100 square meters (1100 square feet). Each level has dimensions of 13.4 m (44 ft) long EW and 7.6 m (25 ft) deep NS. The floor between the two levels lies about 0.6 m (2 ft) below the natural ground level of the site, which was initially completely flat. The ground was scooped out on the south side (and used for berming on the other three sides) to expose 1.5 m (5 ft) of glass on the lower level; this glass starts 0.9 m (3 ft) above the bottom (floor) of the lower level. There are 2.4 m (8 ft) of glass on the upper level.

North of the upper level is a 46-square-meter (500-square-foot) garage-storage area which extends the whole length of the house, serving as a buffer against north and west winds in winter. There is berming on the north, east, and west sides of the house. The lower level is completely below ground level except for the 1.5-square-meter (5-square-foot) glass exposure on the south side. The home is earth-sheltered in the sense that over half its exterior envelope is underground or bermed.

The upper level is very open and serves as the daytime living area; it includes living room, dining room, family room, kitchen, pantry, and bathroom, with only the last two rooms actually separated off by walls. The kitchen is located in the center, and serves as a focal point for the upper level. Persons using the kitchen have an excellent panoramic view of the outdoors through the south glass, and persons in the family and living rooms have similar views.

The lower level includes three bedrooms, one bath, a darkroom, and storage and utility areas. The bedrooms were located on the lower level for tornado protection (provided by the concrete floor above it) and summer coolness (provided by thermal stratification and earth-contact cooling).

To lend architectural interest to the home, an arch was added near the front of the home and the stair tower was allowed to protrude out of the south wall. These features have no energy significance; indeed they slightly degrade the thermal performance of the home since the arch is only insulated to about R-22. The stair tower originally had single-pane curved plastic glazing, which has since been replaced by low-emissivity double-pane glass.


The home has no north windows and only a few east and west windows, which are partly shaded in summer by the ends of the arch. Their total area is about 4.6 square meters (50 square feet).

There are a total of about 37 square meters (400 square feet) of south glass, consisting of double-pane tempered window glass. On the upper level there are seven pieces of fixed glass and one sliding glass door; on the lower level are five pieces of fixed glass and two sliding glass doors. The sliding glass doors have more air infiltration than the fixed windows, but they allow ventilation and (for the two on the lower level) egress. The south glass serves as the direct gain passive solar collector. On a sunny day in mid-winter, if there is a good snow cover to reflect solar radiation, the solar heat admitted through the south glass can be as much as 18 MJ per square meter (1600 Btu per square foot) through the double-pane glass; this corresponds to 670 MJ (640,000 Btu) of heat admitted to the interior, equivalent to 16 hours' output from the home's auxiliary natural gas furnace.

Night insulation is used on most of the south glass during the colder months (typically from December through March, and perhaps a few weeks before and after). The movable insulation originally consisted of 2.5 cm (1 inch) to 5 cm (2 inch) thick panels of Klegecell, a lightweight rigid polyvinyl chloride foam manufactured by the American Klegecell Corporation of Grapevine, Texas; this material was chosen because of its excellent handling properties and the fact that it does not burn on its own like most plastic insulating foams. These panels were cut to fit the individual pieces of glazing, and then covered with a fabric design by Linda Hodges so that they form a large mural when in place. This insulation was typically installed about 5 to 6 p.m. nightly and removed between 7 and 9 a.m. Removal and installation each took about 10 minutes and was no more than a minor inconvenience. During the day the panels were stored in small spaces designed for them on each level. More recently, Window Quilt material was installed on a majority of the windows, and the insulating panels are no longer used. The insulating panels were cheaper, more efficient, but less convenient to use.


The primary thermal storage in the home is the floor between the two levels. It consists of 20 cm (8 inch) thick concrete cored slabs topped by 5 cm (2 inches) of exposed aggregate, for a total mass of over 40 Mg (40 tons). The cores in this floor run NS and serve as the return ducts to the auxiliary natural gas furnace; on sunny days the furnace fan can then be used to help transfer solar heat into the floor, although this is not necessary for good thermal performance. Since the upper surface of this floor is mostly uncovered (only a few small rugs are used on it), and since the lower surface has only a thin plaster texture on it (to give the appearance of an ordinary ceiling), this slab floor is a source of radiant heat to both levels.

In addition to the thermal storage floor, there is a considerable amount of thermal storage on the lower level, which has 20-cm (8-inch) poured concrete walls and a 10-cm (4-inch) slab floor.


This home was superinsulated for the Iowa climate. The bulk of the ceiling has 30 cm (12 inches) of fiberglass insulation and 2.5 cm (1 inch) of polystyrene rigid insulation, amounting to a little over R-40; however, the decorative arch at the front of the house has only half as much insulation. The lower level concrete walls are insulated outside with 7.5 cm (3 inches) of extruded polystyrene rigid insulation. The upper level east and west walls (which support some earth berming) are 2x8 walls (approximately 18 cm thick) insulated to about R-30. The upper level north wall (between the house and the garage-storage area) is a 2x4 wall (approximately 9 cm thick) with rigid insulation added to make it about R-20. The house also has a continuous polyethylene vapor barrier 20 mm (8 mil) thick.


The mechanical system for the home consists of a forced air system connected to the backup natural gas furnace and the fireplace. The furnace has an input of 15 kW (50,000 Btu/h) and a stated output of 12 kW (40,000 Btu/h). The cores in the concrete cored floor are the return ducts to the furnace and serve as supply ducts from the fireplace; this enables the furnace fan to be turned on during sunny winter days to act as an active assist to help transfer heat more quickly into the cored slab floor. The fireplace is in the family room, on the north wall, with its masonry extending into the garage-storage area; it is an energy-efficient fireplace with glass doors and a source of outside combustion air. A blower is used to bring indoor air into the space between the firebox and the fireplace masonry and then force the heated air back into the cores in the concrete floor between the two levels of the home.

The home has a natural gas range in the kitchen area; it has an exhaust fan with a filter to remove cooking odors, but the exhaust is not vented to the exterior of the home. Water is heated with a well-insulated natural gas water heater. The clothes dryer is heated with natural gas, and its exhaust is vented to the outdoors at all times of the year.

Large appliances in the interior include a refrigerator-freezer in the kitchen, a washing machine on the lower level, and a freezer. The freezer was originally located in the pantry, but in December 1983 was moved a short distance into the garage-storage area, at which time a microwave-convection oven was placed in the pantry. The refrigerator-freezer dates to 1970 and consumes 3.5 kWh of electricity daily. The freezer, which uses 2 kWh/day during most of the year, is a replacement for a less-efficient freezer which was determined to use 7 kWh/day.


The home was built on a city lot about 2300 square meters (25,000 square feet) in size, a little over half an acre. The exterior of the home is an "edible landscape" with many types of berries, fruit and nut trees, grapevines, and garden vegetables. The berries include blackberries, blueberries, boysenberries, red and black currants, elderberries, gooseberries, raspberries (black, red, and yellow), and strawberries. The fruit trees include apple, butternut, cherry, peach, pear, plum, and walnut trees. Perennial vegetables include asparagus, chicory, horseradish and rhubarb.

Many perennial and annual flowers are planted on the lot. There were also beehives (so that the flowers could be regarded as "edible"). The owners are active in the Ames Garden Club.


The Hodges Residence was occupied during its first ten years by a family of four (two adults and two children), beginning in September 1979. Records of energy consumption and performance were kept using minimum-maximum thermometers and meters. The natural gas furnace and the water heater both have gas meters to record their consumption, and a special electric meter records the electrical energy consumption of the blowers (fans) associated with the forced air system of the home. In addition, the utilities' outdoor gas and electric meters record total consumption. Finally, all wood burned in the fireplace was weighed on a bathroom scale.

All the meters are read at the end of each month in order to have exact monthly energy consumption. In addition, the records from the utility bills are kept to provide cost information.

In the analysis of the home's energy consumption, it has been assumed that the auxiliary heat actually delivered to the inside of the home is equal to 70 percent of the heat of combustion in the natural gas furnace and 30 percent of the heat of combustion of wood in the fireplace, as well as all the electricity. The heats of combustion have been assumed to be 37 MJ per cubic meter (100,000 Btu/CCF) for the natural gas and 17.5 MJ/kg (7,500 Btu/pound) for wood.

The measured natural gas usage in the Hodges Residence for the ten year period was as follows:

It will be seen that water heating, rather than space heating, was the major use of natural gas in this home. This is characteristic of gas-heated homes in the southern United States (such as Texas or Louisiana) but is extremely unusual in the northern United States. During the ten years the average natural gas bill was $360 annually or $30 monthly for water heating, space heating, cooking, and clothes drying for a family of four.

The measured use of electricity in the home during the ten-year period was as follows:

The total expenditure in this home for energy - natural gas plus electricity - was $9,604, an average of $960 annually or $80 monthly. Comparably-sized but less energy-efficient homes in this part of the United States have energy expenditures that are typically 2 to 3 times as high as they are in the Hodges Residence.

The monthly energy records of the home have been correlated with temperatures during the winter months to determine the Home Heating Index [Ref. 1], which is defined as the total natural gas plus electricity input into the home (excluding furnace flue losses) divided by the product of the interior area of the home and the time-temperature integral for the exterior. The Home Heating Index averaged over the months of December, January and February is a little under 0.7 W/C per square meters (3 Btu/F-day per square foot). At the time the Hodges Residence was built in the later 1970s the average Home Heating Index for homes in Iowa was approximately three times as large and is still more than twice as large.


Although energy conservation receives more attention than power conservation (load management), the latter is extremely important, especially for electric utilities. An electric utility's power plant capacity is determined not by its customers' demands for energy, but by their peak demand load. For most U.S. utilities, including all the major Iowa utilities, this peak demand load occurs on hot summer afternoons when the air conditioning load is high.

Utilities have been experimenting with seasonal and time-of-use pricing in order to encourage the shift of energy consumption from on-peak to off-peak hours, the same strategy employed by the telephone companies. The typical time-of-use pricing by electric utilities allows very low rates (often 30 to 40 percent of on-peak rates) during off-peak hours, which are usually evening hours and weekends. As an example, Iowa Electric Light and Power, which serves many customers in the central Iowa area, defines on-peak as 7 a.m. to 8 p.m. Mondays through Saturdays and off-peak as the rest of the week; it bills customers who voluntarily agree to time-of-use rates at 125 percent of the usual rate for on-peak consumption and only 50 percent of the usual rate for off-peak consumption.

Two significant energy demands of a home - heating and cooling - can be shifted at least partly into off-peak hours by the use of thermal storage mass. For example, the storage could be heated at night, when rates are lowest, and the heat allowed to enter the home during daytime hours, thereby reducing daytime heating. Some utilities sell thermal storage for this purpose; Iowa Electric markets an "Off-Peak Electric Furnace" using a large, well-insulated ceramic brick which is heated at night by electric resistance heating, then used as a heat source by day.

A home with considerable thermal mass on the interior should be able to use the mass naturally for this purpose. Since the Hodges Residence has considerable thermal mass with a large surface area, the concept was tested during the 1980-81 and 1981-82 heating seasons (a mild year and a colder-than-average year with a wicked month of January). The furnace was manually shut off at the thermostat each day at 7 a.m. (Sundays included) and kept off until 8 p.m. On a few occasions, when it was predicted that the following day would be both cold and cloudy, the thermostat was turned up at night (to about 20 C (68 F) in order to store extra heat for the next day.

This experiment was successful: no uncomfortable days were experienced, and indeed the other family members never commented on and were apparently not aware of the daytime furnace shutoff. The combination of internal heat generation (cooking, appliances, and so forth) and some solar gain even on cloudy days was sufficient to prevent the home from cooling down too much by day.


No air conditioning was used in the Hodges Residence during the first ten years. It was predicted (correctly) that the home would be comfortable for all but a few days of the summer through the use of natural cooling methods. These include the following:

There are no overhangs on the south glass. It was originally planned to plant some deciduous trees on the south side of the home. However, after the home was occupied it became clear that it would be undesirable to give up the panorama of the south sky and its views of stars, planets, moon, cloud formation, thunderstorms, and so forth. The only shading devices for the south glazing are canvas shades used on the inside. It should be noted that, contrary to general opinion, unshaded south glass admits far less solar heat in summer than in winter. About 4 or 5 times as much solar heat is received on a clear mid-winter day as on a clear mid-summer day.

During the summer months, the lower level of the home remained comfortably cool, normally between about 21 C (70 F) and 27 C (80 F). At times, especially during humid weather, one or more small electric fans are used to provide a cooling breeze. There are few times when the temperature exceeded 27 C (80 F), and blankets are used in the bedrooms all but a few days of the year.

The upper level is warmer, and electric fans are used more often to provide a breeze. In some years there are no or few days during which the upper level is felt to be uncomfortable (such as one in 1981 and none in 1982), but in other years there may be several dozen. During the unusually hot summer of 1983, for example, there were many days on which the upper level was uncomfortable unless one sat in the breeze of an electric fan; nevertheless, the lower level remained generally comfortable during this period. From the electric utility bills, it can be estimated that in the summer of 1983 the electricity used by the electric fans, including those of the mechanical system, amounted to about 1000 kilowatthours, costing about $80. This is considerably below the operating cost of air conditioning, but the home was not as comfortable as it would have been with air conditioning.

At the conclusion of the ten-year period an efficient central air conditioner was purchased for the home, and is currently in use during hot periods during the summer.


The components of the direct gain passive solar space heating system of the Hodges Residence are just the south glazing and the concrete floors and walls of the building. Since these components are part of the building structure, they do not, strictly speaking, involve an extra cost to the home. As the building costs of the home (about $540 per square meter or $50 per square foot) fell in the normal range for good-quality construction in central Iowa at the time, it is clear that the passive solar design has not significantly affected the cost of construction.

However, while the glass and concrete that constitute the passive solar system are typical construction materials in central Iowa, they are not the cheapest materials available. In particular, they cost more than the alternative of wood frame construction with a normal amount of windows. It has been estimated that the additional expenses relative to more conventional wood frame construction amounted to about $6000, of which $2000 was for extra glazing and $4000 for the extra cost of concrete.

This additional cost of $6000 transcribes into an annual cost of $585 at the mortgage interest rate of 9.75 % per annum. The actual additional cost, taking into account income tax deductions for mortgage interest, is closer to $400 a year. Therefore the annual total of natural gas bills plus electricity bills plus increased mortgage costs amounts to about $1400, considerably lower than the gas and electric bills of comparably-sized homes in this region.

Actually, the estimated annual savings exceed this amount. The useful solar heat received during an average heating season is about 50 to 60 GJ (50 to 60 million Btu); "useful" in this sense refers to useful in providing heat to the interior of the home at times when it contributes to increased environmental comfort for the occupants. The value of this solar heat is about $400 to $500 annually, compared to natural gas at $6 per million Btu used in a furnace with 70 % annual fuel utilization efficiency.

In addition, the direct gain system provides useful daylighting from sunrise to sunset all twelve months of the year, greatly reducing the need for electric lighting in the home. The value of this light for the Hodges family has been estimated as equivalent to an average of 4 kWh per day, or $100 a year since electricity in Ames averages 7 cents a kilowatthour.

In addition, the passive solar design of the home is such that it also reduces the need for air conditioning. It is estimated that if air conditioning were used in the home, the savings would amount to at least $100 a year in operating costs. There are also substantial savings in annual interest costs from not having to purchase a central air conditioning unit for the home; this probably amounts to over $100 a year.

There are some other, subtler economic benefits of the direct gain passive solar system of the home. One is the extra daylighting, which transcribes into less use of electricity for lighting. During the day all parts of the home, except the photographic darkroom and the utility-storage area on the north side of the lower level, are well-lit and require no electric lighting, even on normally cloudy days. Another benefit is that the design, with predominantly south rather than west or east windows, greatly reduces solar heat inputs in summer, which reduce air conditioning bills (or, in the absence of air conditioning, make the home more comfortable than it would otherwise have been).

None of these economic considerations take into account the substantial psychological benefits from the direct gain passive system, which most passive solar home occupants feel equals or exceeds the energy cost benefits. There are a surprising number of homes in Iowa which were built from passive solar home plans but faced in a direction other than south in order to take advantage of a good view; the homeowners liked the design and felt it was affordable, but are not securing the design's energy advantages.


The Hodges Residence has proved extremely pleasant to live in, both in winter and in other months. The direct gain system provides excellent natural lighting to both levels, even on cloudy days. The ability to look outdoors and see the landscape, the wildlife, the weather, the clouds, the sky, and astronomical objects at night is marvelous.

Glare is not a problem, probably for two reasons. First, the whole south wall is glazed and the sidewalls in the home are light-colored, so that there is not the harsh contrast that can occur in homes with only a small amount of glazing. Second, the interior design of the home is such that even when the sun is low in the sky in winter, it will not be in the field of view of the occupants during their normal activities. This might not have been the case if the glazing extended extra high on the south side.

The temperature fluctuations in the home are not large, because of the large amount of thermal storage mass with a large surface area. On cold winter nights the home typically cools down about 5 to 7 Celsius degrees (9 to 13 Fahrenheit degrees), and the daytime swings are not much larger. The home is normally maintained in the range of 18 C to 27 C (65 F to 80 F); below the lower limit the thermostat turns on the auxiliary furnace, and windows are opened when the temperature exceeds the upper limit (as can occur on occasional warm, sunny days in mid-winter).

The masonry floor, which consists of 5 cm (2 inches) of exposed aggregate over the cored slabs, is very nice. Being somewhat uneven in texture and color, it does not show dirt, yet it is easy to clean with just a vacuum cleaner. For a family with two children, it has proved far more practical than a carpeted floor, and there is no desire to return to the wall-to-wall carpets used in the family's previous home. The floor can be walked on barefoot, as it is reasonably smooth and warm. (The kitchen was built with a rubber floor over the concrete slab instead of exposed aggregate.)

The indoor air quality appears to be good. The home was built using as little as possible in the way of particleboard, plastics, and other materials that might contribute pollutants to the air. The clothes dryer is vented to the exterior. The kitchen range has a filter vent which disperses the combustion products throughout the home so that they do not concentrate in the kitchen area; this is satisfactory since the use of the range and oven are not enough to contribute substantial concentrations of combustion products to the house as a whole. The annual average radon concentration on both levels of the home is about 45 becquerels per cubic meter (1.2 picocuries per liter), less than one-third of the action guideline of the U.S. Environmental Protection Agency. The home has low air infiltration compared to conventional homes, but was not ruthlessly tightened. The home is often given extra ventilation by opening of windows during wintertime, yet the home is energy-efficient and has low energy bills.

With regard to indoor air quality, we have noticed that upon coming into the home from the outdoors, the interior never seems stale or musty or smelly, except in rare instances when, for example, something boiled over on the range. Yet in going outside, we have often noted a poorer air quality. This is particularly true on winter nights, when the foul odor of wood-burning systems is often polluting the outdoor air. On one occasion when asphalt was being poured over a parking lot several blocks away and the air outside the home was very foul-smelling, there was only a hint of the asphalt odor indoors.

There are many psychological benefits to living in a direct gain home, and these are felt to be worth as much or more than the energy savings. It is awfully nice, in the middle of a cold Iowa winter, to be able to "sunbathe" in the window on a cold but sunny day, with indoor temperatures above 25 C, watching the overcoated masses outdoors struggling against the elements. While a conventional home might remain for several straight months at 20 C (68 F) (or whatever thermostat setting is used), the direct gain home will often warm up well beyond that, a welcome respite from the cold.


[1] Hodges, L. "The Effect of Internal Gains in Residential Space Heating Analyses." Energy: The International Journal, Vol. 10, 1985, pages 1273-1276.

This description of the Hodges Residence was adapted from a paper entitled "The Hodges Residence: The First Ten Years." published in the International Journal of Ambient Energy.

House in Summer of 1982

Plan of Upper Level

Plan of Lower Level

Copyright 1996 by Laurent Hodges, 12 Physics Hall, Iowa State University, Ames, IA 50011-3160
Laurent Hodges' home page.